Inkjet printer having printhead and ink for minimizing corrosion of exposed corrodible structures within printhead

ABSTRACT

An inkjet printer includes: an inkjet printhead having an exposed corrodible structure containing silicon nitride, borophosphosilicate glass (BPSG) or silicon oxide; and an ink reservoir containing said ink which is in fluid communication with said printhead. The ink includes: water; a dye; and a metal additive for minimizing corrosion of the exposed structure.

FIELD OF THE INVENTION

This invention relates to inkjet inks for inkjet printers. It has beendeveloped primarily for minimizing corrosion of corrodible structures inprintheads by dye-based inkjet inks.

CROSS REFERENCES

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

6,755,509 7,222,943 7,188,419 7,168,166 7,086,719 12/246,332 12/246,3367,246,886 7,128,400 7,108,355 6,991,322 7,287,836 7,118,197 10/728,7847,364,269 7,077,493 6,962,402 10/728,803 7,147,308 10/728,779 7,118,1987,168,790 7,172,270 7,229,155 6,830,318 7,195,342 7,175,261 7,465,0357,108,356 7,118,202 10/773,186 7,134,744 7,134,743 7,182,439 7,210,7687,465,036 7,134,745 7,156,484 7,118,201 7,111,926 7,431,433 7,401,9017,468,139 11/744,885 11/097,308 7,328,978 7,334,876 7,147,306 7,448,7347,425,050 11/014,764 11/014,763 7,331,663 7,360,861 7,328,973 7,427,1217,407,262 7,303,252 7,249,822 11/014,762 7,311,382 7,360,860 7,364,2577,390,075 7,350,896 7,429,096 7,384,135 7,331,660 7,416,287 11/014,7377,322,684 7,322,685 7,311,381 7,270,405 7,303,268 7,470,007 7,399,0727,393,076 11/014,750 11/014,749 7,249,833 11/014,769 11/014,7297,331,661 11/014,733 7,300,140 7,357,492 7,357,493 11/014,766 7,380,9027,284,816 7,284,845 7,255,430 7,390,080 7,328,984 7,350,913 7,322,6717,380,910 7,431,424 7,470,006 11/014,732 7,347,534 7,441,865 7,469,9897,367,650 7,469,990 11/688,863 11/688,864 11/688,865 7,364,26511/741,766 12/014,767 12/014,768 12/014,769 12/014,770 12/014,77112/014,772 12/049,371 12/049,372 12/049,373 12/146,399 12/334,51912/339,039 12/557,517 12/613,404 12/546,682 12/062,514 7,887,1707,416,280 6,902,255 6,623,101 6,406,129 6,505,916 6,457,809 6,550,8956,457,812 11/607,976 7,735,970 7,901,046 7,794,613 7,938,974 7,568,78711/946,840 12/323,471 12/508,564 12/142,779 12/142,782

BACKGROUND OF THE INVENTION

The present Applicant has developed a plethora of thermal bubble-formingprintheads and thermal bend-actuated printheads. The Applicant's thermalbubble-forming printheads include those with suspended heater elements(as described in, for example, U.S. Pat. No. 6,755,509; U.S. Pat. No.7,246,886; U.S. Pat. No. 7,401,910; and U.S. Pat. No. 7,658,977, thecontents of which are incorporated herein by reference) and those withembedded heater elements (as described in, for example, U.S. Pat. No.7,377,623; U.S. Pat. No. 7,431,431; US 2006/250453; and U.S. Pat. No.7,491,911, the contents of which are incorporated herein by reference).The Applicant's thermal bend-actuated printheads typically have moveablepaddles defined in a nozzle plate of the printhead (as described in, forexample, U.S. Pat. No. 7,926,915; U.S. Pat. No. 7,669,967; and US2011/0050806, the contents of which are incorporated herein byreference).

One characteristic of many of the Applicant's inkjet printheads is anozzle chamber and/or a nozzle plate comprised of silicon nitride. Thenozzle plate spans across an ink ejection face of the printhead anddefines a roof of each nozzle chamber in the printhead. It will beappreciated that the roof of each nozzle chamber, as well as chambersidewalls, have surfaces that are continuously in contact with inkcontained in each nozzle chamber. In some printheads, the nozzle chamberroof may comprise a bi-layer of silicon nitride and silicon oxide (see,for example, U.S. Pat. No. 7,658,977 and US 2011/0050806). In theseprintheads, the lower silicon nitride layer has a surface which isexposed to ink in the nozzle chamber. In other printheads (e.g. U.S.Pat. No. 6,755,509), the nozzle chamber roof may consist of a mono-layerof silicon nitride.

Silicon nitride is an excellent material for use in fabricating nozzlechambers and nozzle plates in inkjet printheads. Silicon nitride hasexcellent mechanical robustness under high pressures, good resistance tocracking and can be deposited by PECVD, which is compatible withconventional MEMS fabrication techniques.

Notwithstanding the excellent mechanical properties of silicon nitridenozzle plates, the present Applicant has observed an apparent corrosionof silicon nitride when exposed to certain dye-based inks over prolongedperiods (e.g. 6-12 months). In the Applicant's bi-layered nozzle platestructures, comprising a lower layer of silicon nitride and an upperlayer of silicon oxide, an unacceptable degree of roof delamination hasbeen observed. This delamination results in a significant reduction inprint quality, especially in black ink channels where delamination ismost severe. Further SEM investigation of delaminated printheadsrevealed that the silicon nitride layer of each roof had apparentlycorroded whereas the silicon oxide layer was left relatively intact.However, some corrosion of silicon oxide was also observed, albeit at arelatively slower rate of corrosion than silicon nitride.

Another characteristic of the Applicant's inkjet printheads is theintegration MEMS and CMOS layers in a single printhead integratedcircuit (IC), which has enabled the development of inkjet printheadshaving a high nozzle density using standard semiconductor fabricationtechniques. In the Applicant's printhead ICs, it is necessary for ink toflow from a backside of the printhead IC, which receives ink from amolded ink manifold, to a frontside of the printhead IC containing theMEMS nozzle chambers. Therefore, the ink must pass through ink inletsdefined in the CMOS layers. Clearly, if ink comes into contact with anyCMOS Metal layers then this is potentially catastrophic for printheadoperation.

In a typical CMOS design, a lowermost Metal 1 CMOS layer is disposed ona BPSG layer. This BPSG layer has an edge exposed to ink via the inkinlet. Although many inks do not affect this BPSG layer (or othersilicon oxide layers) in the printhead IC, it has been found that someinks cause significant corrosion of the BPSG layer which is problematicfor printhead longevity. Relatively slower corrosion of a CVD oxideinterlayer dielectric was also observed in some instances.

From the foregoing, it will be apparent that there are a number ofstructures in printhead, which are potentially corrodible by exposure todye-based inks at typical pHs (e.g. pH 6-8).

One possible solution to the corrosion problems discussed above, whichis currently under investigation by the present Applicant, is tophysically isolate the silicon nitride or BPSG layer from the ink. Forexample, the nozzle plate may be designed so that a protective barrierlayer is disposed between the silicon nitride and the ink. Alternativelya protective collar may be formed around the inner surfaces of each inkinlet. However, this type of mechanical solution to the problem ofcorrosion has the significant drawback that it requires a more complexprinthead design, as well as the development and optimization ofsuitable MEMS fabrication processes. On a practical level, redesigningprintheads is highly undesirable, especially when optimized printheadfabrication processes are well-established and suitable formass-production.

It would therefore be desirable to seek an alternative solution to theproblem of corrosion in printheads by dye-based inks, which does notrequire modifying the design of the printhead.

US 2010/0271448 describes dissolution of silicates from elementalsilicon in printheads and identifies high pH pigment-based inks as thecause of this dissolution. US 2010/0271448 proposes the use of trivalentaluminium for passivating native silicon surfaces and suppressing thedissolution of silicates caused by high pH pigment-based inks.Consistent with the present Applicant's observations, US 2010/0271448reports that no dissolution of silicates is observed when siliconprinthead dies are exposed to dye-based inks, and the addition oftrivalent aluminium, therefore, has no effect in such systems.

SUMMARY OF THE INVENTION

In a first aspect, there is an inkjet printer comprising:

an inkjet printhead comprising at least one corrodible structurecomprised of silicon nitride, borophosphosilicate glass (BPSG) orsilicon oxide, the corrodible structure having a surface exposed to ink;and

an ink reservoir containing the ink, the ink reservoir being in fluidcommunication with the printhead,

wherein the ink comprises: water; a dye; and a metal additive forminimizing corrosion of the exposed surface, the metal additivecomprising one or more metals selected from the group consisting of:Al(III), Fe(III), Fe(II), Cu(II), Cu(I), Bi(III), Cr(III), Mg(II),Sr(II), Ba(II), Ce(III), Ag(I), Al, Fe, Ce, Cu, Cr, Mg and Ag.

As used herein, the term “silicon nitride” refers to any ceramicmaterial comprised of silicon and nitrogen. Typically, silicon nitrideis deposited by PECVD and may, therefore, contain other elements, suchas hydrogen and oxygen, depending on the precise deposition conditions.Nevertheless, it will be understood by those skilled in the art thatceramic materials containing, for example, silicon, nitrogen andhydrogen are all within the ambit of the term “silicon nitride”.Equally, it will be understood by those skilled in the art that ceramicmaterials containing, for example, silicon, nitrogen and oxygen(sometimes referred to as silicon oxynitrides) are all within the ambitof the term “silicon nitride”. Usually, silicon and nitrogen aretogether the primary component of silicon nitride, accounting for atleast 50 wt. %, at least 70 wt. % or at least 90 wt. % of the ceramicmaterial referred to herein as “silicon nitride”.

Borophosphosilicate glass (BPSG) is well known to those skilled in thesemiconductor art. BPSG is a type of silicate glass that includesadditives of both boron and phosphorus. It is typically used as aninsulating layer in CMOS.

As used herein, the term “silicon oxide” refers to any oxide of silicondeposited using CVD. Such oxides may also be referred to in the art as“CVD oxide”. Some examples of source gases for depositing silicon oxideinclude: silane, and oxygen; dichlorosilane and nitrous oxide; andtetraethylorthosilicate (TEOS).

The Applicant's initial studies found that seemingly robust siliconnitride nozzle structures were corroded upon prolonged exposure tocertain dye-based inks. Further studies found that corrosion of a BPSGlayer of CMOS and relatively slower corrosion of silicon oxide layerswas caused by prolonged exposure to certain dye-based inks.

It was then determined that this corrosion was inhibited by trace metaladditives in the ink, such as Al(III) and Fe(III). The corrosionmechanism and the involvement of trace metal additives in inhibitingthis corrosion is not fully understood. Initially, the present inventorssuspected that impurities in the inks were the culprit. For example,trace amounts of fluoride present in the printhead (either as a residuefrom etching processes or as a ubiquitous ink contaminant) couldpotentially catalyze the corrosion of Si—N bonds in silicon nitride.Furthermore, it was plausible that sequestration of fluoride by thetrace metal additive would inhibit this corrosion.

However, further experimentation was unable to confirm definitively thepresence of fluoride ions being responsible for silicon nitridecorrosion. With more detailed studies of BPSG corrosion, a clearerpicture emerged of a potential corrosion mechanism. A key finding wasthat certain dyes appeared to be far more corrosive towards BPSG thanother dyes. Specifically, sulfonated dyes having an ammonium, potassiumor sodium counterions were found to be much more corrosive thancorresponding dyes having a lithium or tetramethylammonium (TMA)counterion. In general, the order of corrosivity appeared to be:

ammonium>K>Na>Li>TMA

In particular, potassium counterions were identified as beingparticularly problematic, because sulfonated dyes having potassiumcounterions are present in many commercially-available inkjet dyes andwere found to be very corrosive towards silicon nitride and BPSGstructures in the Applicant's printheads.

The present inventors have postulated that the order of corrosivity maybe related to the solubility of salts generated at the surfaces ofcorrodible structures. For example, BPSG surfaces liberate smallquantities of phosphate and borate anions, which potentially combinewith cations from the inkjet dye to form various salts. Relativelyinsoluble salts, such as lithium phosphate and tetramethylammoniumphosphate tend to form a passivating layer, which protects the BPSGsurface from further corrosion. On the other hand, relatively solublesalts, such as potassium phosphate will transport the liberatedphosphate away from the BPSG surface and accelerate corrosion.

A similar passivating mechanism via insoluble salt formation may also beoperating with silicon nitride. This is consistent with the observationthat less soluble dye cations (e.g. lithium) tend to be much lesscorrosive towards silicon nitride than more soluble dye cations (e.g.potassium).

Regardless of the actual mechanism of corrosion, it was found,remarkably, that the addition certain metals, such as Al(III) andFe(III), to corrosive dye-based inks had a dramatic effect in reducingthe rates of corrosion. For the most corrosive inks, a larger quantityof metal additive was required (e.g. 10 to 100 ppm) to inhibit corrosioncompletely, while for the less corrosive inks a smaller quantity ofmetal additive was required (e.g. 0.5 to 5 ppm).

A range of metal additives were tested and a correlation emerged betweenthe solubility of phosphate salts and the rate of corrosion. In general,metal ions having relatively insoluble phosphate salts were found toinhibit corrosion of BPSG. Based on these observations, it is understoodthat the following metal ions (having relatively insoluble phosphates)are suitable for inhibiting corrosion in printheads: Al(III), Fe(III),Fe(II), Cu(II), Cu(I), Bi(III), Cr(III), Mg(II), Sr(II), Ba(II),Ce(III), Ag(I). Likewise, some metals are suitable for inhibitingcorrosion in their elemental form e.g. Al, Fe, Ce, Cu, Cr, Mg and Ag.However, trivalent metal additives, such as Al(III) and Fe(III) aregenerally preferred.

Typically, the metal additive is soluble in the dye-based ink. Someexamples of suitable soluble metal additives include: aluminium nitratenonahydrate; aluminium perchlorate nonahydrate; aluminium chloratenonahydrate; iron(III) nitrate nonahydrate; iron(III) hydroxide,ammonium iron(III) sulfate, iron (III) sulfate heptahydrate, iron(III)chloride and iron(III) bromide; copper(II) nitrate; magnesium nitratehexahydrate; bismuth(III) nitrate pentahydrate; and silver nitrate.

In cases where the metal additive is present in the ink in an insolubleparticulate form (e.g. alumina particles, aluminium metal particlesetc.), the average particle size of the metal additive is typically inthe range of 0.01 to 2 microns, 0.05 to 1 microns or 0.1 to 0.5 microns.

Optionally, the ink comprises 0.01 to 25 wt. % of the dye, or optionally0.1 to 10 wt. %. The balance of the ink is an ink vehicle, thecomponents of which are not particularly limited. Some exemplary inkvehicles are described hereinbelow.

Optionally, the dye comprises is a sulfonated dye comprising one or moresulfonate groups having a counterion. For example, the dye may be asulfonated azo dye (e.g. Food Black 2, Direct Blue 1), a sulfonatedphthalocyanine dye or other sulfonated dye. A more detailed descriptionof suitable inks and dyes is provided hereinbelow.

Optionally, the sulfonated dye comprises at least one of: an ammonium, apotassium or a sodium counterion. Some common inkjet dyes comprisingpotassium counterions have been shown to be particularly aggressiveagainst corrodible silicon nitride and BPSG structures, when comparedto, for example, dyes containing only lithium counterions.

The amount of metal additive required to suppress corrosion will varydepending on how aggressive a particular ink is towards corrodiblestructures. Optionally, the metal additive is contained in the ink in aconcentration in the range of 0.1 to 200 ppm with respect to the metal.Optionally, the concentration of metal additive is in the range of 0.1to 150 ppm, 0.1 to 100 ppm or 0.5 to 50 ppm. It is surprising thatrelatively low concentrations of a soluble metal additive (e.g.aluminium nitrate) are efficacious in suppressing relatively high ratesof corrosion in silicon nitride and BPSG structures. With an insolublemetal additive (e.g. alumina), a higher concentration of metal additivemay be required. For example, a concentration of 0.1 to 1 g/L istypically required for alumina to suppress silicon nitride corrosion.

Optionally, the ink has an alkaline pH. Optionally, the ink has a pH inthe range of 7.5 to 9.5, or optionally in the range of 8 to 9. It hasbeen found that metal additives are particularly efficacious when usedwith relatively alkaline inkjet inks.

Optionally, the printhead is selected from the group consisting of:thermal bubble-forming inkjet printheads; thermal bend-actuated inkjetprintheads; and piezoelectric inkjet printheads.

Optionally, each nozzle chamber comprises a roof and sidewalls, whereinat least one of the roof and sidewalls is comprised of silicon nitride.

Optionally, each roof defines part of a nozzle plate for the printhead,wherein the nozzle plate is comprised of silicon nitride.

Optionally, each nozzle chamber comprises a floor and a roof having aplurality of layers, and wherein a lower layer of the roof is comprisedof silicon nitride.

Optionally, the roof is bilayered, and wherein an upper layer of theroof is comprised of silicon oxide.

Optionally each nozzle chamber comprises a heater element for heating atleast some of the ink to a temperature sufficient to cause formation ofa bubble in the nozzle chamber.

Optionally, each nozzle chamber comprises a thermal bend actuatorcomprising a passive beam and a thermoelastic active beam fused to thepassive beam.

Optionally, the passive beam comprises a first layer comprised ofsilicon nitride and a second layer comprised of silicon dioxide, thesecond layer being sandwiched between the first layer and the activebeam.

Optionally, a roof of each nozzle chamber comprises a moveable paddle,the moveable paddle comprising the thermal bend actuator.

Optionally, the printhead comprises CMOS layers having at least onelayer of BPSG exposed to the ink.

Optionally, the printhead comprises CMOS layers having at least onelayer of silicon oxide exposed to ink.

Optionally, the printhead comprises a plurality of ink inlets defined byopenings through the CMOS layers. The ink inlets may expose edgeportions of BPSG and/or silicon oxide layer to ink.

In a second aspect, there is provided a kit comprising:

a printer having an inkjet printhead, the inkjet printhead comprising atleast one structure comprised of silicon nitride, borophosphosilicateglass (BPSG) or silicon oxide, the structure having a surface exposed toink; and

at least one ink cartridge for installation in the printer, the inkcartridge containing an inkjet ink, wherein the ink comprises:

water;

a dye; and

a metal additive for minimizing corrosion of the exposed surface, themetal additive comprising one or more metals selected from the groupconsisting of: Al(III), Fe(III), Fe(II), Cu(II), Cu(I), Bi(III),Cr(III), Mg(II), Sr(II), Ba(II), Ce(III), Ag(I), Al, Fe, Ce, Cu, Cr, Mgand Ag.

The kit according to third aspect may be in the form of a box or thelike containing the printer and a plurality of ink cartridges forinstallation in the printer by a user.

In a third aspect, there is provided a method of minimizing corrosion ofat least one corrodible structure in a microfluidic device, thecorrodible structure being comprised of silicon nitride,borophosphosilicate glass (BPSG) or silicon oxide, the method comprisingexposing a surface of the structure to a liquid comprising a metaladditive, wherein the metal additive comprising one or more metalsselected from the group consisting of: Al(III), Fe(III), Fe(II), Cu(II),Cu(I), Bi(III), Cr(III), Mg(II), Sr(II), Ba(II), Ce(III), Ag(I), Al, Fe,Ce, Cu, Cr and Ag.

Microfluidic devices will be well-known to the person skilled in theart. Usually, the microfluidic device is an inkjet printhead, but itwill be appreciated that the present invention is equally applicable toother microfluidic devices, such as lab-on-a-chip devices. The presentApplicant has described lab-on-a-chip devices fabricated using MEMStechnology in, for example, U.S. Pat. No. 7,887,756, the contents ofwhich is herein incorporated by reference.

Optionally, the microfluidic device is an inkjet printhead selected fromthe group consisting of: thermal bubble-forming inkjet printheads;thermal bend-actuated inkjet printheads; and piezoelectric inkjetprintheads.

Optionally, the liquid protects the exposed corrodible surfaces againstsubsequent corrosion by conventional inkjet inks lacking the metaladditive. Hence, the method according to the fourth aspect may beemployed as a treatment for a printhead prior to distribution and use.This obviates continuous exposure of the printhead to the metal additivevia a treated ink, which may have unforeseen deleterious side-effectsover time.

Optionally, the liquid is a test ink supplied to the printhead forinitial testing and verification of the printhead following fabricationof the printhead. The test ink is typically removed from the printheadprior to distribution and use.

Optionally, the liquid is a shipping fluid which is used to fill fluidicpathways in the printhead prior to shipment of the printhead. Theshipping fluid is typically utilized to minimize degradation ofhydrophilically-treated surfaces in the printhead during shipment (e.g.surfaces exposed to an oxygen plasma). However, in accordance with thisembodiment, the shipping fluid may function to dope the printhead withthe metal additive and inhibit subsequent degradation of corrodiblestructures in the printhead. The shipping fluid is removed from theprinthead prior to use.

Optionally, the liquid is infused with the metal additive by virtue ofexposure to a metal surface en route to the corrodible surfaces. Themetal surface may be any surface in the fluidic pathway of a printer orother microfluidic device. For example, any one of an ink cartridge, anink line, a pressure-regulating chamber, an ink manifold orchannels/chambers in the printhead itself may comprise the metalsurface. Typically, the metal surface is comprised of aluminium.

In a fifth aspect, there is provided an inkjet printer comprising:

an inkjet printhead comprising at least one corrodible structurecomprised of silicon nitride, borophosphosilicate glass (BPSG) orsilicon oxide, the corrodible structure having a surface exposed to ink;and

a fluidic pathway for delivering ink to nozzle openings in theprinthead, wherein part of the fluidic pathway is comprised of aluminiummetal such that ink supplied to the nozzle openings is exposed to thealuminium metal.

The printer according to the fifth aspect advantageously enablesconventional inkjet inks (i.e. those inks not specifically formulatedwith a metal additive) to be used in printheads having corrodiblestructures, whilst still minimizing corrosion of such structures.

Optionally, the aluminium metal is disposed in one or more of:

an ink cartridge in fluid communication with the printhead;

a pressure-regulating chamber in fluid communication with the printhead;

an ink manifold for supplying ink to the printhead;

an ink line for supplying ink to the printhead;

an inline filter;

an inline pump;

ink supply channels defined in the printhead; and

the nozzle chambers of the printhead.

Optionally, a surface area of the aluminium metal to which the ink isexposed is sufficient to infuse the ink with an aluminium additive.

Optionally, the aluminium additive is present in ink downstream of thealuminium metal in an amount ranging from 0.01 to 200 ppm with respectto aluminium.

Optionally, a surface area of the aluminium metal to which the ink isexposed is in the range of 0.5 to 200 cm². Optionally, the surface areaof aluminium is in the range of 10 to 150 cm².

Optionally, at least part of the aluminium metal is configured as a meshor a sponge in order to maximize its surface area.

Optionally, the fluidic pathway comprises an ink reservoir for supplyingink to the printhead, wherein the ink reservoir contains a conventionalinkjet ink formulated without an metal additive.

In a sixth aspect, there is provided an ink cartridge for an inkjetprinter, the ink cartridge having one or more surfaces exposed to an inkcontained therein, wherein at least of one of the surfaces is defined byaluminium metal.

The ink cartridge according to the fifth aspect advantageously enablesconventional inkjet inks (i.e. those inks not specifically formulatedwith a metal additive) to be used in printheads having corrodiblestructures, whilst still minimizing corrosion of such structures.

Typically, the ink contained in the ink cartridge is a dye-based ink asdescribed above in connection with the first aspect.

In a seventh aspect, there is provided an inkjet ink comprising:

water;

0.01 to 25 wt. % of a dye having one or more sulfonate groups, whereinsaid dye comprises one or more counterions selected from the groupconsisting of: potassium, sodium and ammonium; and

a metal additive for minimizing corrosion of corrodible surfaces in aninkjet printhead, said metal additive comprising one or more metalshaving a metal phosphate solubility of less than 1 gram per liter.

Optionally, the metal additive comprises one or more metals having ametal phosphate solubility of less than 0.1, less than 0.01, less than0.001 or less than 10⁻⁵ grams per liter (e.g. a metal phosphatesolubility in the range of 10⁻⁹ to 0.1 grams per liter)

Optionally, the metal additive comprises one or more metals selectedfrom the group consisting of: Al(III), Fe(III), Fe(II), Cu(II), Cu(I),Bi(III), Cr(III), Mg(II), Sr(II), Ba(II), Ce(III), Ag(I), Al, Fe, Ce,Cu, Cr, Mg and Ag.

Optionally, the dye comprises a potassium counterion.

Optionally, the dye has no carboxylate groups.

Optionally, the ink has pH in the range of 6 to 8.

Optionally, the ink further comprises 5 wt % to 40 wt % of a co-solvent.Typical co-solvent systems are described in further detail below.

Optionally, the co-solvent comprises one or more water-soluble organiccompounds selected from the group consisting of: N—(C₁₋₆alkyl)-2-pyrrolidinone; C₁₋₆ alcohol; ethylene glycol; diethyleneglycol; and glycerol.

Optionally, the ink further comprises one or more of: a surfactant; a pHadjuster; a biocide; and a sequestering agent. Examples of such inkcomponents are described in further detail below.

In an eighth aspect, there is provided a use of an inkjet ink asdescribed in connection with the seventh aspect for inhibiting corrosionof a corrodible structure in a printhead, the corrodible structure beingcomprised of silicon nitride, borophosphosilicate glass (BPSG) orsilicon oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, inwhich:

FIG. 1 is a perspective view of part of a thermal bubble-forming inkjetprinthead having a silicon nitride nozzle plate and chamber sidewalls;

FIG. 2 is a side view of one of the nozzle assemblies shown in FIG. 1,including CMOS layers;

FIG. 3 is a perspective of the nozzle assembly shown in FIG. 2;

FIG. 4 is a perspective view of part of a thermal bubble-forming inkjetprinthead having a bi-layered nozzle plate;

FIG. 5 is a perspective view of a nozzle assembly having a thermal bendactuator with a bi-layered passive beam;

FIG. 6 is a perspective view a partially-fabricated nozzle assemblyshown in FIG. 5 showing the active thermoelastic beam;

FIG. 7 is perspective view of an inkjet print engine;

FIG. 8 is a schematic view of a fluidics system incorporating exposedaluminium; and

FIG. 9 is a side view of a thermal bubble-forming inkjet nozzleincorporating a layer of aluminium.

DETAILED DESCRIPTION OF THE INVENTION

As foreshadowed above, the present invention minimizes corrosion ofcorrodible structures in inkjet printheads which are exposed to certaindye-based inks. The corrodible structures are typically silicon nitride,BPSG and, to a lesser extent, silicon oxide.

Silicon Nitride Corrosion

Initially, the present Applicant observed print defects in certain colorchannels of its inkjet printheads, most noticeably in black colorchannels. Forensic examination of such printheads using SEM microscopyrevealed that a degree of roof delamination was occurring and it wasposited that the dye-based ink was responsible for corroding siliconnitride structures in the nozzle chamber. Significantly, in printheadshaving a bilayered roof (an upper layer of silicon oxide and a lowerlayer of silicon nitride), the corrosion was most evident in the siliconnitride layer. Silicon surfaces in the printhead were observed to beresistant to corrosion by the dye-based inks.

Following these observations, the Applicant conducted a comprehensiveseries of experiments in order to elucidate more fully the nature of thecorrosion and to find possible solutions to this problem.

To this end, silicon nitride coupons were fabricated having a layer ofsilicon nitride deposited onto a blanket silicon substrate. Thethickness of silicon nitride was accurately measured before and aftersoaks tests in various ink formulations using a UV interferometer. Inthis way, the corrosiveness of a range ink formulations towards siliconnitride could be determined.

It was found that the rate of corrosion was strongly dependent on theparticular ink formulation. Those inks containing potassium salts ofsulfonate groups (e.g. Food Black 2) were found to be highly corrosive.Corresponding sodium salts were still corrosive but to a lesser extentthan potassium salts. These observations were broadly consistent withSEM observations on actual printheads.

Initial experiments showed that aluminium additives in the ink (e.g.aluminium nitrate nonahydrate, alumina, elemental aluminium etc.) wereremarkably effective in suppressing silicon nitride corrosion inotherwise highly corrosive inks. In many cases, even at very lowconcentrations (e.g. 10 ppm), the aluminium additive was highlyeffective in suppressing corrosion of silicon nitride Therefore, it wasconcluded that treatment of dye-based inks with a small concentration ofan aluminium additive (e.g. aluminium nitrate nonahydrate) was aneffective method of minimizing the roof delamination observed in theApplicant's printheads.

Iron additives in the form of soluble iron(III) salts were also shown toreduce the rates of silicon nitride corrosion, although aluminiumadditives were generally more effective in most inks. Combinations ofaluminium and iron additives were, likewise, effective in suppressingsilicon nitride corrosion.

It was further observed that the corrosive effects of ‘untreated’ inkscould be suppressed merely by exposing these inks to an aluminium metalsurface. Presumably, a trace amount of an aluminium additive is infusedinto the ink by the aluminium surface, which is sufficient to suppresscorrosion of silicon nitride. These observations have importantimplications for the design of inkjet printers. If an aluminium metalsurface is incorporated into the ink pathway upstream of the printhead,then untreated inks may be used in the Applicant's printers withoutcorroding silicon nitride structures therein. It will be appreciatedthat an aluminium surface may be readily incorporated anywhere into theink pathway, for example, in ink cartridges, ink lines, filters,pressure-regulating chambers or even the printhead itself.

BPSG Corrosion

With the silicon nitride corrosion results in hand, the Applicant theninvestigated another known failure mechanism in its inkjetprintheads—that is, corrosion of a BPSG layer in the CMOS layers. Duringthe course of printhead testing, it had become evident from SEMmicroscopy that an exposed edge region of a BPSG layer was corroded andled to electrical failure once ink was allowed to reach CMOS metallayer(s).

Accordingly, BPSG test coupons were fabricated by analogy with thesilicon nitride coupons described above. A layer of BPSG was depositedonto a blanket silicon substrate and the thickness of BPSG layer wasaccurately measured before and after soaks tests in various inkformulations using a UV interferometer. In this way, the corrosivenessof a range ink formulations towards BPSG could be determined.

Once again, it was found that the rate of corrosion of BPSG was stronglydependent on the particular ink formulation. Those inks containingpotassium salts of sulfonate groups were found to be highly corrosive.On the other hand, inks having a metal counterion which formedrelatively insoluble phosphates (e.g. lithium) were found to be muchless corrosive. In virtually all cases, the addition of an aluminiumand/or iron additive (in the form of a soluble Al(III) or Fe(III) salt)was effective in suppressing the BPSG corrosion.

Other metal additives, such as copper, bismuth, magnesium and silversalts were also shown to reduce the rate of BPSG corrosion. Typically,in order to suppress a rate of BPSG corrosion the metal additivecomprises one or more metals wherein the metal has a corresponding metalphosphate solubility of less than 1 gram per liter. The most effectivemetals for suppressing BPSG corrosion rates had a corresponding metalphosphate solubility of less than 10⁻⁵ grams per liter.

Silicon Oxide Corrosion

The Applicant's studies have shown that silicon oxide appears to corrodeat a relatively slower rate than either silicon nitride or BPSG.Nevertheless, silicon oxide structures are still somewhat corrodibleupon prolonged exposure to certain dye-based inks and it has been found,through suitable silicon oxide coupon testing, that this relatively slowcorrosion can also be suppressed using the metal additives describeabove.

A detailed description of the Applicant's experiments demonstrating theeffects of the present invention are presented in the ExperimentalSection hereinbelow.

Inkjet Printheads Comprising Exposed Corrodible Structures

The inkjet inks described herein minimize corrosion of exposedcorrodible structures in inkjet printheads. There now follows a briefdescription of some examples of inkjet printheads, which incorporatesuch corrodible structures.

Thermal Bubble-Forming Inkjet Printheads

Referring to FIG. 1, there is shown part of printhead comprising aplurality of nozzle assemblies as described in U.S. Pat. No. 7,303,930,the contents of which is herein incorporated by reference. FIGS. 2 and 3show one of these nozzle assemblies in side-section and cutawayperspective views.

Each nozzle assembly comprises a nozzle chamber 24 formed by MEMSfabrication techniques on a silicon wafer substrate 2. The nozzlechamber 24 is defined by a roof 21 and sidewalls 22 which extend fromthe roof 21 towards the silicon substrate 2. As shown in FIG. 1, eachroof is defined by part of a nozzle plate 56, which spans across anejection face of the printhead. The nozzle plate 56 and sidewalls 22 areformed of the same material, which is deposited by PECVD over asacrificial scaffold of photoresist during MEMS fabrication. The nozzleplate 56 and sidewalls 21 are formed only of silicon nitride in theprinthead shown in FIGS. 1 to 3. Silicon nitride is chosen, because itis readily deposited by PECVD and has the characteristics of hardness,robustness and resistance to cracking. Moreover, the inherently relativehydrophilic nature of silicon nitride is advantageous for supplying inkto the nozzle chambers 24 by capillary action. However, SEM microscopyhas revealed corrosion of these silicon nitride structures in some colorchannels of used printheads.

Returning to the details of the nozzle chamber 24, it will be seen thata nozzle opening 26 is defined in a roof of each nozzle chamber 24. Eachnozzle opening 26 is generally elliptical and has an associated nozzlerim 25. The nozzle rim 25 assists with drop directionality duringprinting as well as reducing, at least to some extent, ink flooding fromthe nozzle opening 26. The actuator for ejecting ink from the nozzlechamber 24 is a heater element 29 positioned beneath the nozzle opening26 and suspended across a pit 8. Current is supplied to the heaterelement 29 via electrodes defined by an exposed region of a Metal 4 CMOSlayer 9A. In these regions, the Metal 4 CMOS layer 9A is exposed througha passivation layer 10 covering the underlying CMOS layers. The pit 8 isdefined in a CVD oxide layer 5 positioned beneath the uppermost Metal 4CMOS layer 9A.

When a current is passed through the heater element 29, it rapidlysuperheats surrounding ink to form a gas bubble, which forces inkthrough the nozzle opening 26. By suspending the heater element 29, itis completely immersed in ink when the nozzle chamber 24 is primed. Thisimproves printhead efficiency, because less heat dissipates into theunderlying substrate 2 and more input energy is used to generate abubble.

As seen most clearly in FIG. 1, the nozzles are arranged in rows and anink supply channel 27, which extends longitudinally along the row,supplies ink to each nozzle in the row. The ink supply channel 27delivers ink to an ink inlet passage 15, which, in turn, supplies ink toan ink conduit 23 extending parallel with the nozzle rows. The inkconduit 23 supplies ink into a side of each nozzle chamber 24.

Returning to FIGS. 2 and 3, the ink inlet passage 15 is defined by anopening through the CMOS layers and an upper portion of the siliconsubstrate 2. The CMOS layers are comprised of a lower BPSG layer 11 andfour metal layers 9A, 9B, 9C, 9D which are separated from each other bydielectric CVD oxide layers 5. It will be noted that the BPSG layer 11and the CVD oxide layers 5 have edge portions defining sidewalls of theink inlet passage 15. Hence, these layers define potentially corrodiblestructures which are exposed to ink flowing through the ink inletpassage 15. SEM microscopy of used printheads has revealed notches inthe BPSG layer 11 as a result of corrosion; these notches can eventuallygrow to expose the Metal 1 CMOS layer 9D, resulting in printheadfailure.

The MEMS fabrication process for manufacturing such printheads wasdescribed in detail in U.S. Pat. No. 7,303,930, the contents of whichare herein incorporated by reference.

The operation of printheads having suspended heater elements isdescribed in detail in the Applicant's U.S. Pat. No. 7,278,717, thecontents of which are incorporated herein by reference.

The Applicant has also described thermal bubble-forming inkjetprintheads having embedded heater elements. Such printheads aredescribed in, for example, U.S. Pat. No. 7,246,876 and US 2006/0250453,the contents of which are herein incorporated by reference. It will beappreciated that the advantages of the present invention are realizedirrespective of whether the heater element is suspended or embedded inthe nozzle chamber.

Referring to FIG. 4, there is shown a printhead having a bilayerednozzle plate. A lower layer 101A of the nozzle plate is comprised ofsilicon nitride and an upper layer 101B of the nozzle plate is comprisedof silicon oxide. All other features of the printhead shown in FIG. 4are the same as the printhead shown in FIG. 1, and it will beappreciated that all like structures have been given the same referencenumerals in FIGS. 1 to 4.

The fabrication and advantages of printheads haying a bilayered nozzleplate are described in U.S. Pat. No. 7,658,977, the contents of whichare herein incorporated by reference. However, when the printhead shownin FIG. 4 is exposed to certain dye-based inks, a degree of roofdelamination is observed due to corrosion of the silicon nitride layer101A in the uppermost corners of each nozzle chamber.

Thermal Bend-Actuated Inkjet Printheads

Referring to FIG. 5, there is shown a nozzle assembly 100 for a thermalbend-actuated printhead, as described in US 2011/0050806, the contentsof which is incorporated herein by reference.

The nozzle assembly 100 is comprised of a substrate 101 havingelectrodes 102 formed in an upper portion thereof. (For clarity, theCMOS layers and passivation layer are not shown in FIG. 5). Theelectrode 102 shown in FIG. 5 is one of a pair of adjacent electrodes(positive and earth) for supplying power to a thermoelastic active beam110 disposed on a roof of the nozzle chamber 105 via connector posts108. The connector posts 108 extend linearly between the electrodes 102and the active beam 110, and the electrodes receive power from CMOSdrive circuitry (not shown) in upper layers of the substrate 101.

The connector posts 108 are encased in sidewalls 104 of the nozzlechamber 105. The sidewalls may be comprised of silicon oxide or siliconnitride. As shown in FIG. 5, the sidewalls are comprised of siliconnitride.

A bilayered roof of the nozzle chamber 105 is comprised of a lower layerof silicon nitride 107 and an upper layer of silicon oxide 106. Part ofthe roof defines a passive beam 116 for the thermoelastic beam 110disposed on the roof (see FIG. 6).

The thermoelastic beam 110 and bilayered passive beam 116 togetherdefine a thermal bend actuator. Upon actuation, the thermoelastic beam110 expands relative to the passive beam 116 causing a moveable part ofthe roof to bend towards the substrate 110 resulting in ejection of inkfrom the nozzle opening 113.

Silicon nitride is employed in the passive beam 116, because it is lesssusceptible to cracking than silicon oxide and allows a greater range ofresidual stresses—both compressive and tensile stresses. Furthermore,silicon nitride is completely impermeable, which minimizes nozzlefailure via leaching of ions from ink in the nozzle chamber to theactive beam 110. Since silicon nitride has a much higher thermalconductivity than silicon oxide, the passive beam 116 employs aninsulating layer of silicon oxide between the silicon nitride and thethermoelastic active beam 110. However, it will be appreciated that theexposed silicon nitride layer in the passive beam 116 is potentiallycorrodible by certain dye-based inks as described herein.

The thermoelastic active beam member 110 may be comprised of anysuitable thermoelastic material, such as titanium nitride, titaniumaluminium nitride or aluminium alloys. As explained in the Applicant'searlier US Publication No. 2008/0129793 (the contents of which areherein incorporated by reference), vanadium-aluminium alloys are apreferred material, because they combine the advantageous properties ofhigh thermal expansion, low density and high Young's modulus.

Still referring to FIG. 5, a polymer coating 80 covers the roof of thenozzle assembly 100. The polymer coating 80 extends over the entirenozzle plate of the printhead and provides a hydrophobic ink ejectionsurface. The polymer layer 80 also fills a perimeter region around amoveable part of the roof to provide a mechanical seal for the moveablepart of the roof. The polymer has a sufficiently low Young's modulus toallow the actuator to bend towards the substrate 101 during actuation.

The polymer coating 80 is typically comprised of a polymerized siloxane,which may be deposited in a thin layer (e.g. 0.5 to 2.0 microns) using aspin-on process. Examples of suitable polymeric materials arepoly(alkylsilsesquioxanes), such as poly(methylsilsesquioxane);poly(arylsilsesquioxanes), such as poly(phenylsilsesquioxane); andpoly(dialkylsiloxanes), such as a polydimethylsiloxane. It will beappreciated that the printheads described in connection with FIGS. 1 to4 may comprise a polymer coating 80 to provide a desirably hydrophobicink ejection surface.

Other Microfluidic Devices Comprising Exposed Silicon Nitride Structures

Although the present invention has been developed for use in some of theApplicant's MEMS inkjet printheads, as described above, it will beappreciated that the invention is not so limited in scope.

Piezoelectric printheads may include corrodible structures (e.g. siliconnitride, BPSG) and the present invention is equally applicable to suchprintheads. An example of a piezo printhead incorporating an exposedsilicon nitride surface is described in U.S. Pat. No. 4,992,808,assigned to Xaar Limited.

Other microfluidic devices, such as lab-on-a-chip devices useful for theanalysis of biological fluids, may also include silicon nitride or BPSGstructures. It will be appreciated that such devices will also benefitfrom the methods of minimizing corrosion described herein. By way ofexample, the Applicant's microfluidic devices described in U.S. Pat. No.7,887,756, the contents of which are herein incorporated by reference,may comprise exposed silicon nitride surfaces.

Inks

The inks employed in connection with the present invention typicallycomprise 0.01-25 wt. % of a dye, a metal additive additive and an inkvehicle as the balance. The amount of metal additive may vary dependingon the type of additive present. For example, soluble trivalentaluminium may be present in an amount ranging from 0.01 to 200 ppm. Onthe other hand, insoluble metal additives (e.g. alumina particles,elemental aluminium particles etc) may be present in the ink in largeramounts as described herein.

Inkjet dyes will be well-known to the person skilled in the art and thepresent invention is not limited to any particular type of dye. By wayof example, dyes suitable for use in the present invention include azodyes (e.g. Food Black 2), metal complex dyes, naphthol dyes,anthraquinone dyes, indigo dyes, carbonium dyes, quinone-imine dyes,xanthene dyes, cyanine dyes, quinoline dyes, nitro dyes, nitroso dyes,benzoquinone dyes, naphthoquinone dyes, phthalocyanine dyes (includingnaphthalocyanine dyes), and metal phthalocyanine dyes (including metalnaphthalocyanine dyes, such as those described in U.S. Pat. No.7,148,345, the contents of which is herein incorporated by reference).

Examples of suitable dyes include: CI Direct Black 4, 9, 11, 17, 19, 22,32, 80, 151, 154, 168, 171, 194 and 195; CI Direct Blue 1, 2, 6, 8, 22,34, 70, 71, 76, 78, 86, 142, 199, 200, 201, 202, 203, 207, 218, 236 and287; CI Direct Red 1, 2, 4, 8, 9, 11, 13, 15, 20, 28, 31, 33, 37, 39,51, 59, 62, 63, 73, 75, 80, 81, 83, 87, 90, 94, 95, 99, 101, 110, 189,225 and 227; CI Direct Yellow 1, 2, 4, 8, 11, 12, 26, 27, 28, 33, 34,41, 44, 48, 86, 87, 88, 132, 135, 142 and 144; CI Food Black 1 and 2; CIAcid Black 1, 2, 7, 16, 24, 26, 28, 31, 48, 52, 63, 107, 112, 118, 119,121, 172, 194 and 208; CI Acid Red 4, 14, 18, 23, 27, 73, 87, 88, 114,131, 138, 151; CI Acid Blue 1, 7, 9, 15, 22, 23, 27, 29, 40, 43, 55, 59,62, 78, 80, 81, 90, 102, 104, 111, 185 and 254; CI Acid Yellow 1, 3, 4,7, 11, 12, 13, 14, 19, 23, 25, 34, 38, 41, 42, 44, 53, 55, 61, 71, 76and 79; CI Reactive Blue 1, 2, 3, 4, 5, 6, 7, 8, 9, 13, 14, 15, 17, 18,19, 20, 21, 25, 26, 27, 28, 29, 31, 32, 33, 34, 37, 38, 39, 40, 41, 43,44 and 46; CI Reactive Red 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 15, 16,17, 19, 20, 21, 22, 23, 24, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39,40, 41, 42, 43, 44, 45, 46, 49, 50, 58, 59, 63, 64, and 180; CI ReactiveYellow 1, 2, 3, 4, 6 7, 11, 12, 13, 14, 15, 16, 17, 18, 22, 23, 24, 25,26, 27, 37 and 42; CI Reactive Black 1, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14and 18; Pro-Jet® dyes available from Fuji Film Imaging Colorants, Inc.(e.g. Pro-Jet® Cyan 1; Pro-Jet® Cyan GLF; Pro-Jet® Fast Cyan 2; Pro-Jet®Fast Black 1; Pro-Jet® Fast Black 2; Pro-Jet® Black 168; Pro-Jet®Magenta 1; Pro-Jet® Magenta 432; Pro-Jet® Fast Magenta 2; Pro-Jet®Violet 631; Pro-Jet® Yellow 1; Pro-Jet® Fast Yellow 2; Pro-Jet® Yellow1G; Pro-Jet® Yellow 746; and Pro-Jet® Yellow 492).

The present invention is particularly efficacious when used inconnection with sodium or potassium salts of sulfonated dyes (e.g. FoodBlack 2). Such dyes, and potassium salts in particular, have been shownto be very aggressive towards exposed corrodible structures in theprinthead, such as silicon nitride nozzle roofs and the BPSG layer inCMOS.

Ink vehicles for inkjet inks will be well known to the person skilled inthe art and the ink vehicles used in the present invention are notparticularly limited. The present Applicant has recently describednon-aqueous inkjet inks for thermal inkjet printheads (see U.S.application Ser. No. 12/577,517 filed on Sep. 11, 2009, the contents ofwhich are herein incorporated by reference), and such non-aqueous inksare also within the ambit of some aspects of the present invention.Non-aqueous ink vehicles for use in thermal inkjets typically comprise aN—(C₁₋₆ alkyl)-2-pyrrolidinone (e.g. N-methyl-2-pyrrolidinone) and aC₁₋₆ alcohol (e.g. ethanol).

However, the ink vehicles used in the present invention are typicallyconventional aqueous ink vehicles comprising at least 40 wt % water, atleast 50 wt % water or at least 60 wt % water. Usually, the amount ofwater present in the inkjet ink is in the range of 50 wt % to 90 wt %,or optionally in the range of 60 wt % to 80 wt %.

Aqueous inkjet inks compositions are well known in the literature and,in addition to water, may comprise other components, such as co-solvents(including humectants, penetrants, wetting agents etc.), surfactants,biocides, sequestering agents, pH adjusters, viscosity modifiers, etc.

Co-solvents are typically water-soluble organic solvents. Suitablewater-soluble organic solvents include C₁₋₄ alkyl alcohols, such asethanol, methanol, butanol, propanol, and 2-propanol; glycol ethers,such as ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether, ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol mono-n-propyl ether, ethylene glycolmono-isopropyl ether, diethylene glycol mono-isopropyl ether, ethyleneglycol mono-n-butyl ether, diethylene glycol mono-n-butyl ether,triethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butylether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol,propylene glycol monomethyl ether, propylene glycol monoethyl ether,propylene glycol mono-t-butyl ether, propylene glycol mono-n-propylether, propylene glycol mono-isopropyl ether, dipropylene glycolmonomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycolmono-n-propyl ether, dipropylene glycol mono-isopropyl ether, propyleneglycol mono-n-butyl ether, and dipropylene glycol mono-n-butyl ether;formamide, acetamide, dimethyl sulfoxide, sorbitol, sorbitan, glycerolmonoacetate, glycerol diacetate, glycerol triacetate, and sulfolane; orcombinations thereof.

Other useful water-soluble organic solvents, which may be used asco-solvents, include polar solvents, such as 2-pyrrolidone,N-methylpyrrolidone, ε-caprolactam, dimethyl sulfoxide, sulfolane,morpholine, N-ethylmorpholine, 1,3-dimethyl-2-imidazolidinone andcombinations thereof.

The inkjet ink may contain a high-boiling water-soluble organic solventas a co-solvent, which can serve as a wetting agent or humectant forimparting water retentivity and wetting properties to the inkcomposition. Such a high-boiling water-soluble organic solvent includesone having a boiling point of 180° C. or higher. Examples of thewater-soluble organic solvent having a boiling point of 180° C. orhigher are ethylene glycol, propylene glycol, diethylene glycol,pentamethylene glycol, trimethylene glycol, 2-butene-1,4-diol,2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, tripropylene glycolmonomethyl ether, dipropylene glycol monoethyl glycol, dipropyleneglycol monoethyl ether, dipropylene glycol monomethyl ether, dipropyleneglycol, triethylene glycol monomethyl ether, tetraethylene glycol,triethylene glycol, diethylene glycol monobutyl ether, diethylene glycolmonoethyl ether, diethylene glycol monomethyl ether, tripropyleneglycol, polyethylene glycols having molecular weights of 2000 or lower,1,3-propylene glycol, isopropylene glycol, isobutylene glycol,1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol,glycerol, erythritol, pentaerythritol and combinations thereof.

Other suitable wetting agents or humectants include saccharides(including monosaccharides, oligosaccharides and polysaccharides) andderivatives thereof (e.g. maltitol, sorbitol, xylitol, hyaluronic salts,aldonic acids, uronic acids etc.)

The inkjet ink may also contain a penetrant, as one of the co-solvents,for accelerating penetration of the aqueous ink into the recordingmedium. Suitable penetrants include polyhydric alcohol alkyl ethers(glycol ethers) and/or 1,2-alkyldiols. Examples of suitable polyhydricalcohol alkyl ethers are ethylene glycol monomethyl ether, ethyleneglycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycolmonomethyl ether acetate, diethylene glycol monomethyl ether, diethyleneglycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethyleneglycol mono-isopropyl ether, diethylene glycol mono-isopropyl ether,ethylene glycol mono-n-butyl ether, diethylene glycol mono-n-butylether, triethylene glycol mono-n-butyl ether, ethylene glycolmono-t-butyl ether, diethylene glycol mono-t-butyl ether,1-methyl-1-methoxybutanol, propylene glycol monomethyl ether, propyleneglycol monoethyl ether, propylene glycol mono-t-butyl ether, propyleneglycol mono-n-propyl ether, propylene glycol mono-isopropyl ether,dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether,dipropylene glycol mono-n-propyl ether, dipropylene glycolmono-isopropyl ether, propylene glycol mono-n-bulyl ether, anddipropylene glycol mono-n-butyl ether. Examples of suitable1,2-alkyldiols are 1,2-pentanediol and 1,2-hexanediol. The penetrant mayalso be selected from straight-chain hydrocarbon diols, such as1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,7-heptanediol, and 1,8-octanediol. Glycerol may also be used as apenetrant.

Typically, the amount of co-solvent present in the ink is in the rangeof about 5 wt % to 40 wt %, or optionally 10 wt % to 30 wt %. A specificexample of a co-solvent system, which may be used in the presentinvention, comprises ethylene glycol, 2-pyrrolidone and glycerol.

The inkjet ink may also contain one or more surface active agents(“surfactant”), such as an anionic surface active agent, a zwitterionicsurface active agent, a nonionic surface active agent or mixturesthereof. Useful anionic surface active agents include sulfonic acidtypes, such as alkanesulfonic acid salts, α-olefinsulfonic acid salts,alkylbenzenesulfonic acid salts, alkylnaphthalenesulfonic acids,acylmethyltaurines, and dialkylsulfosuccinic acids; alkylsulfuric estersalts, sulfated oils, sulfated olefins, polyoxyethylene alkyl ethersulfuric ester salts; carboxylic acid types, e.g., fatty acid salts andalkylsarcosine salts; and phosphoric acid ester types, such asalkylphosphoric ester salts, polyoxyethylene alkyl ether phosphoricester salts, and glycerophosphoric ester salts. Specific examples of theanionic surface active agents are sodium dodecylbenzenesulfonate, sodiumlaurate, and a polyoxyethylene alkyl ether sulfate ammonium salt.

Examples of zwitterionic surface active agents includeN,N-dimethyl-N-octyl amine oxide, N,N-dimethyl-N-dodecyl amine oxide,N,N-dimethyl-N-tetradecyl amine oxide, N,N-dimethyl-N-hexadecyl amineoxide, N,N-dimethyl-N-octadecyl amine oxide andN,N-dimethyl-N-(Z-9-octadecenyl)-N-amine oxide.

Examples of nonionic surface active agents include ethylene oxide adducttypes, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenylethers, polyoxyethylene alkyl esters, and polyoxyethylene alkylamides;polyol ester types, such as glycerol alkyl esters, sorbitan alkylesters, and sugar alkyl esters; polyether types, such as polyhydricalcohol alkyl ethers; and alkanolamide types, such as alkanolamine fattyacid amides. Specific examples of nonionic surface active agents areethers such as polyoxyethylene nonylphenyl ether, polyoxyethyleneoctylphenyl ether, polyoxyethylene dodccylphenyl ether, polyoxyethylenealkylallyl ether, polyoxyethylene oleyl ether, polyoxyethylene laurylether, and polyoxyalkylene alkyl ethers (e.g. polyoxyethylene alkylethers); and esters, such as polyoxyethylene oleate, polyoxyethyleneoleate ester, polyoxyethylene distearate, sorbitan laurate, sorbitanmonostearate, sorbitan mono-oleate, sorbitan sesquioleate,polyoxyethylene mono-oleate, and polyoxyethylene stearate. Acetyleneglycol surface active agents, such as2,4,7,9-tetramethyl-5-decyne-4,7-diol; ethoxylated2,4,7,9-tetramethyl-5-decyne-4,7-diol; 3,6-dimethyl-4-octyne-3,6-diol or3,5-dimethyl-1-hexyn-3-ol, may also be used. Specific examples ofnonionic surfactants, which may be used in the present invention, areSurfynol® 465 and Surfynol® 440 (available from Air Products andChemicals, Inc)

The surfactant(s) are typically present in the aqueous inkjet ink in anamount ranging from 0.1 wt % to 10 wt %, or optionally in the range of0.2 wt % to 5 wt %.

The aqueous inkjet ink may also include a pH adjuster or buffer, such assodium hydroxide, potassium hydroxide, lithium hydroxide, sodiumcarbonate, sodium hydrogencarbonate, potassium carbonate, potassiumhydrogencarbonate, lithium carbonate, sodium phosphate, potassiumphosphate, lithium phosphate, potassium dihydrogenphosphate, dipotassiumhydrogenphosphate, sodium oxalate, potassium oxalate, lithium oxalate,sodium borate, sodium tetraborate, potassium hydrogenphthalate, andpotassium hydrogentartrate; ammonia; and amines, such as methylamine,ethylamine, diethylamine, trimethylamine, triethylamine,tris(hydroxymethyl)aminomethane hydrochloride, triethanolamine,diethanolamine, diethylethanolamine, triisopropanolamine,butyldiethanolamine, morpholine, propanolamine,4-morpholineethanesulfonic acid and 4-morpholinepropanesulfonic acid.The pH adjuster or buffer may be present in the aqueous inkjet ink in anamount ranging from 0.01 to 5 wt. %, or optionally 0.05 to 1 wt. %.

The aqueous inkjet ink may also include a biocide, such as benzoic acid,dichlorophene, hexachlorophene, sorbic acid, hydroxybenzoic esters,sodium dehydroacetate, 1,2-benthiazolin-3-one (“Proxel® GXL”, availablefrom Arch Chemicals, Inc.), 3,4-isothiazblin-3-one or4,4-dimethyloxazolidine. The biocide may be present in the aqueousinkjet ink in an amount ranging from 0.01 to 5 wt. %, or optionally 0.05to 1 wt. %.

The aqueous inkjet ink may also contain a sequestering agent, such asethylenediaminetetraacetic acid (EDTA). The sequestering agent may bepresent in the aqueous inkjet ink in an amount ranging from 0.01 to 5wt. %, or optionally 0.05 to 1 wt. %.

Experimental Section

The following experiments demonstrate the corrosiveness of certaindye-based inks with respect to silicon nitride, BPSG and silicon oxide.The experiments further demonstrate the advantageous effects of metaladditives in suppressing this corrosion.

1. Silicon Nitride Corrosion Methodology

Silicon nitride of the exact composition, formulation and thickness of atypical printhead roof structure was deposited onto blanket siliconwafers without an oxide overcoat. The wafer was diced into 10 mm×10 mmcoupons and each was rinsed with DI water and ethanol then dried withcompressed air. The thickness of the nitride layer was measuredprecisely three times at the centre of each coupon using a Nanometrics210 UV interferometer Film thickness System. A refractive index of 2.00was used for all measurements and typically a consistent value of within+/−10 Angstroms was recorded. Coupons were placed with the nitride layeruppermost in a soak vessel into which was carefully added 20 g of testfluid. The soak vessels were made of plastic (polystyrene sample jars),because initial experiments using soda glass jars and scintiallationvials gave appreciable background corrosion rates, even with deionizedwater. Ensuring the tile remained face up, the vessel was sealed andplaced in an oven at 70° C. After a period of time, typically ˜100 hrs,the vessels were removed from the oven and cooled to room temperature.The coupons were retrieved, rinsed with DI water and ethanol then driedwith compressed air. The thickness of the nitride layer at the centre ofthe coupon was again measured precisely three times using the sameinterferometer. Differences in film thicknesses were calculated and acorrosion rate, expressed in Angstroms per hour, was obtained bydividing the average thickness of the film lost by the duration of thesoak test in hours. The corrosion rate for three coupons was measuredfor each fluid of interest.

For tests requiring aluminium, squares of 0.3 mm thick aluminium foilmeasuring 10 mm×10 mm were placed in the vessel close to the coupon.

1.1 Colored Ink Silicon Nitride Corrosion Rates

Initially, a range of colored ink formulations were tested for theirsilicon nitride corrosion rates. The inks had the followingformulations:

TABLE A Black Ink 1 Components Batch wt % Diethylene Glycol 122-Pyrrolidone 8 Glycerol 5 Tris(hydroxymethyl)methylamine 0.2 Proxel ®GXL 0.2 Surfynol ® 465 0.2 Direct Black 19 6 Water Balance

TABLE B Cyan Ink 1 Components Batch wt % Diethylene Glycol 122-Pyrrolidone 8 Glycerol 5 Tris(hydroxymethyl)methylamine 0.2 Proxel ®GXL 0.2 Surfynol ® 465 0.2 Acid Blue 9 5 Water Balance

TABLE C Magenta Ink 1 Components Batch wt % Diethylene Glycol 122-Pyrrolidone 8 Glycerol 5 Tris(hydroxymethyl)methylamine 0.2 Proxel ®GXL 0.2 Surfynol ® 465 0.2 Acid Red 52 5 Water Balance

TABLE D Yellow Ink 1 Components Batch wt % Diethylene Glycol 122-Pyrrolidone 8 Glycerol 5 Tris(hydroxymethyl)methylamine 0.2 Proxel ®GXL 0.2 Surfynol ® 465 0.2 Pro-Jet ® Yellow 746 4 Water Balance

TABLE E Black Ink 2 Components Batch wt % Ethylene Glycol 102-Pyrrolidone 7 Glycerol 3 Diethylene Glycol 5Tris(hydroxymethyl)methylamine 0.2 Proxel ® GXL 0.2 Surfynol ® 465 0.2Food Black 2 5 Water Balance

All inks formulated had a pH in the range of 6-8.

All dyes contained a mixture counterions, which include sodium andpotassium ions.

Table 1 shows the corrosion rates for the five inks tested. Deionizedwater (DIW) was also tested by way of a control.

TABLE 1 Silicon Nitride Corrosion Rates for Colored Inks Test No. TestFluid Corrosion Rate (Angstroms per hr) 1 Black Ink 1 10.74 2 Cyan Ink 121.86 3 Magenta Ink 1 0.13 4 Yellow Ink 1 2.04 5 Black Ink 2 34.62 6 DIW0.1

From Table 1, it can be seen that Black Ink 1, Cyan Ink 1 and Black Ink2 are the most corrosive towards silicon nitride. Yellow Ink 1 had amoderate corrosion rate while the magenta ink and deionized water didnot corrode silicon nitride by any appreciable amount. These resultswere broadly consistent with SEM observations of printheads, in whichcyan and black ink channels appeared to show the most corrosion of thesilicon nitride roof layer.

1.2 Effect of pH on Silicon Nitride Corrosion Rates

The effect of pH on silicon nitride corrosion rates was investigated.Accordingly, the black ink and deionized water were tested at pH10 (bythe addition of ammonium hydroxide) and pH1 (by the addition ofhydrochloric acid). Table 2 shows the corrosions rates at these extremepHs (Test Nos. 1 and 6 from Table 1 have been included for comparison).

TABLE 2 Effect of pH on Silicon Nitride Corrosion Rates Test No. TestFluid Corrosion Rate (Angstroms per hr) 1 Black Ink 1 10.74 7 Black Ink1 @ pH 10 20.58 8 Black Ink 1 @ pH 1 0.13 6 DIW 0.01 9 DIW @ pH 10 11.9510 DIW @ pH 1 0.01

As expected, the high pH test fluids were very corrosive towards siliconnitride whereas the low pH test fluids were not corrosive at all.

1.3 Effect of Soluble Al(III) on Silicon Nitride Corrosion Rates

Table 3 shows the corrosions rates for Black Ink 1 spiked with varyingamounts of aluminium by the addition of water-soluble aluminium nitratenonahydrate (Test No. 1 from Table 1 has been included for comparison).

TABLE 3 Effect of Soluble Al(III) Ions on Silicon Nitride CorrosionRates for Black Ink 1 Corrosion Rate Test No. Test Fluid (Angstroms perhr) 1 Black Ink 1 10.74 17 Black Ink 1 + 0.01 ppm Al 7.64 18 Black Ink1 + 0.1 ppm Al 1.50 19 Black Ink 1 + 1 ppm Al 0.05 20 Black Ink 1 + 10ppm Al 0.05 21 Black Ink 1 + 100 ppm Al −0.63

As can be seen from the data in Table 1, the addition of solubletrivalent aluminium ions to Black Ink 1 surprisingly produces a dramaticeffect in reducing the corrosion rate of silicon nitride. At an Alconcentration of 1 ppm or more, corrosion of silicon nitride iscompletely inhibited. Even at lower concentrations of Al, the rate ofcorrosion is markedly diminished.

With this positive result using Black Ink 1, the other colored inksdescribed above were tested to investigate the effects of addingaluminium nitrate nonahydrate on corrosion rates. Table 4 shows the testresults for cyan, magenta and yellow inks (Test Nos. 2, 3 and 4 fromTable 1 have been included for comparison).

TABLE 4 Effect of Soluble Al(III) Ions on Silicon Nitride CorrosionRates for Cyan, Magenta and Yellow Inks Corrosion Rate Test No. TestFluid (Angstroms per hr) 2 Cyan Ink 1 21.86 22 Cyan Ink 1 + 0.1 ppm Al8.23 23 Cyan Ink 1 + 1 ppm Al 0.12 24 Cyan Ink 1 + 5 ppm Al −0.01 25Cyan Ink 1 + 10 ppm Al 0.06 26 Cyan Ink 1 + 50 ppm Al 0.14 27 Cyan Ink1 + 100 ppm Al −0.24 3 Magenta Ink 1 0.13 28 Magenta Ink 1 + 0.1 ppm Al−0.05 29 Magenta Ink 1 + 1 ppm Al 0.12 30 Magenta Ink 1 + 10 ppm Al−0.14 4 Yellow Ink 1 2.04 31 Yellow Ink 1 + 0.1 ppm Al 0.04 32 YellowInk 1 + 0.1 ppm Al 0.01 33 Yellow Ink 1 + 0.1 ppm Al −0.02

As evidenced by the data in Table 5, the corrosiveness of all inkstested was minimized by the addition of relatively small quantities ofaluminium.

The effect of aluminium nitrate spiking was also investigated on theparticularly corrosive Black Ink 2. Hitherto, Black Ink 2 had beenconsidered as potentially too corrosive towards silicon nitride to beused in printheads. Table 5 shows the test results for Black Ink 2, bothuntreated and spiked with aluminium nitrate nonahydrate.

TABLE 5 Effect of Soluble Al(III) Ions on Silicon Nitride CorrosionRates for Black Ink 2 Corrosion Rate Test No. Test Fluid (Angstroms perhr) 5 Black Ink 2 34.62 34 Black Ink 2 + 1 ppm Al 34.04 35 Black Ink 2 +10 ppm Al 25.50 36 Black Ink 2 + 100 ppm Al −0.18

Although, compared to other inks tested, higher quantities of aluminiumwere required in order to fully inhibit silicon nitride corrosion, Table5 demonstrates the effectiveness of aluminium spiking even in highlycorrosive inks.

Given the positive results from Al(III), it was anticipated that otherAl(III) sources would be effective in reducing the rate of siliconnitride corrosion. Alternative Al(III) sources include common alum andaluminium sulfate. Other Al(III) sources will be readily apparent to theperson skilled in the art.

1.4 Effect of Other Additives on Silicon Nitride Corrosion Rates

Soluble borax (sodium tetraborate decahydrate), insoluble alumina andsoluble iron were investigated as alternative additives for inhibitingcorrosion of silicon nitride. The test results for borax, alumina andiron are shown in Tables 6, 7 and 8, respectively. (Test Nos. 1, 2 and 5from Table 1 have been included for comparison, where appropriate).

TABLE 6 Effect of Borax on Silicon Nitride Corrosion Rates Test No. TestFluid Corrosion Rate (Angstroms per hr) 1 Black Ink 1 10.74 37 Black Ink1 + 1 ppm B 15.10 38 Black Ink 1 + 10 ppm B 9.28 39 Black Ink 1 + 100ppm B 12.07 5 Black Ink 2 34.62 40 Black Ink 2 + 1 ppm B 31.12 41 BlackInk 2 + 10 ppm B 35.58 42 Black Ink 2 + 100 ppm B 47.65

TABLE 7 Effect of Alumina on Silicon Nitride Corrosion Rates CorrosionRate Test No. Test Fluid (Angstroms per hr) 1 Black Ink 1 10.74 43 BlackInk 1 + 0.001 g/l Al 16.01 44 Black Ink 1 + 0.01 g/l Al 14.61 45 BlackInk 1 + 0.1 g/l Al 14.41 46 Black Ink 1 + 1 g/l Al 3.14

TABLE 8 Effect of Iron on Silicon Nitride Corrosion Rates Corrosion RateTest No. Test Fluid (Angstroms per hr) 2 Cyan Ink 1 33.16 47 Cyan Ink1 + 100 ppm Fe 6.06 (ammonium iron sulfate) 48 Cyan Ink 1 + 100 ppm Fe6.04 (iron sulfate heptahydrate) 49 Cyan Ink 1 + 100 ppm Fe 4.20 (ironchloride) 50 Cyan Ink 1 + 100 ppm Fe 5.39 (iron bromide)

Table 6 demonstrates that boron in the form of borax apparently has noeffect in suppressing silicon nitride corrosion.

Table 7 demonstrates that alumina has some effect in reducing siliconnitride corrosion, albeit only in relatively high quantities (about 1gram per liter).

Table 8 demonstrates that Fe(III) is effective in suppressing siliconnitride corrosion, irrespective of the iron salt used as the additive.However, Fe(III) is generally not as effective as Al(III) in suppressingsilicon nitride corrosion.

Mixtures of 100 ppm Fe(III) and 100 pm Al(III) were also found to behighly effective in reducing the rate of silicon nitride corrosion,presumably due to the presence of Al(III). Notably, the inhibitingeffects of Al(III) were not diminished by the presence of Fe(III).

1.5 Effect of Aluminium Metal on Silicon Nitride Corrosion Rates

Whilst spiking inks with aluminium (e.g. aluminium nitrate) is anattractive means for inhibiting silicon nitride corrosion, theeffectiveness of such low quantities of aluminium led the presentApplicant to investigate aluminium foil as a possible means forinhibiting silicon nitride corrosion. It was considered that aluminiumfoil could infuse sufficient quantities of Al(III) ions into the ink inorder to suppress silicon nitride corrosion. Such an approach wouldpotentially obviate the need to formulate customized inks spiked withaluminium, or at least provide an alternative to these customized inks.

Various test fluids were immersed in the plastic soak vessel togetherwith aluminium foil, in accordance with the methodology described above.Table 9 shows the results of these tests. (Test Nos. 1 and 5 from Table1 have been included for comparison).

TABLE 9 Effect of Aluminium Foil on Silicon Nitride Corrosion RatesCorrosion Rate Test No. Test Fluid (Angstroms per hr) 1 Black Ink 110.74 51 Black Ink 1 + 10 cm² Al foil 0.03 52 Black Ink 1 + 100 cm² Alfoil 0.01 5 Black Ink 2 34.62 53 Black Ink 2 + 10 cm² Al foil 31.45 54Black Ink 2 + 100 cm² Al foil −0.16

The data presented in Table 9 demonstrate that exposure of test fluidsto aluminium metal is highly effective in inhibiting corrosion ofsilicon nitride. For example, inks formulated with elemental aluminiumparticles arc very effective in minimizing corrosiveness.

More significantly, the data presented in Table 9 has ramifications forthe design of inkjet printers and cartridges. If ink is exposed to analuminium surface upstream of a printhead, then this ink will haveminimal corrosiveness towards silicon nitride in the printhead, even ifit is an ‘untreated’ ink (i.e. an ink not specifically formulated withany aluminium additives).

There are potentially many different parts of a printer's fluidicpathway where an aluminium surface may be incorporated. For example,aluminium may be incorporated into an ink cartridge, inklines/couplings, inline filter(s), pump(s), a pressure-regulatingchamber positioned between the ink cartridge and the printhead, an inkmanifold for delivering ink to inlets of the printhead, or the printheaditself (e.g. an aluminium layer in each nozzle chamber, which can bedeposited by PECVD during MEMS fabrication).

Although a laminar sheet of aluminium foil was employed in Test Nos.51-54, it will be appreciated that the aluminium may have any suitableconfiguration provided that is exposed to the ink. For example, analuminium mesh or sponge may be preferred in some instances formaximizing a surface area of aluminium exposed to the ink.

1.6 Inkjet Printer Fluidics System Incorporating Exposed Aluminium Metal

Inkjet printers incorporating the Applicant's inkjet printheads aredescribed in, for example, U.S. Pat. No. 7,201,468; U.S. Pat. No.7,360,861; U.S. Pat. No. 7,380,910; and U.S. Pat. No. 7,357,496, thecontents of each of which are herein incorporated by reference.

FIG. 7 shows a thermal inkjet printer comprising a print engine 203, asdescribed in Applicant's U.S. application Ser. No. 12/062,514, thecontents of which is herein incorporated by reference. The printerincludes a removable print cartridge 202, comprising a pagewidthprinthead, and a bank of user-replaceable ink cartridges 228. Each colorchannel typically has its own ink reservoir 228 and a correspondingpressure-regulating chamber 206 for regulation of a hydrostatic pressureof ink supplied to the printhead. Hence, the printer has five inkreservoirs 228 and five corresponding pressure-regulating chambers 206.Typical color channel configurations for this five-channel print engine203 are CMYKK or CMYK(IR). Each ink cartridge 228 may comprise an inkjetink as described herein.

Although fluidic connections between the various components are notshown in FIG. 7, it will be appreciated that these connections are madewith suitable hoses in accordance with the fluidics system described in,for example, U.S. application Ser. No. 12/062,514.

FIG. 8 shows schematically a fluidics system 200 of the printer shown inFIG. 7. Several components of the fluidics system 200 have been modifiedto incorporate aluminium, which is exposed to ink delivered by thesystem to the print cartridge 202.

Referring then to FIG. 8, the pressure-regulating chamber 206 suppliesink 204 to an ink inlet 208 of the print cartridge 202 via an upstreamink line 234. The pressure-regulating chamber 206 is positioned belowthe print cartridge 202 and maintains a predetermined set level 210 ofink therein by means of a float valve. 216 The pressure-regulatingchamber 206 includes a layer of aluminium 292, which is exposed to theink 204 contained in the chamber.

Ink 204 is supplied to the pressure-regulating chamber 206 by the inkreservoir 228 positioned at any height h above the set level 210. Theink reservoir 228 is typically a user-replaceable ink tank or inkcartridge, which connects with an ink supply line 230 when installed inthe printer. The ink supply line 230 provides fluidic communicationbetween the ink reservoir 228 and an inlet port of thepressure-regulating chamber 206.

The ink reservoir 228 comprises an aluminium sponge 290, which providesa large surface area of aluminium exposed to ink contained therein. Ofcourse, other configurations of aluminium (e.g. sheet, mesh etc) areequally possible.

The printhead cartridge 202 shown in FIG. 8 also has an ink outlet 236,which is connected to a downstream ink line 238. The downstream ink line238 is connected to a return port of the chamber 206 and comprises aninline ink pump 240 and filter 282. The filter 282 may comprise analuminium mesh, which is exposed to ink returned to the chamber 202.Equally, the ink pump 240 may comprise aluminium exposed to the ink 204.

From the foregoing, it will be appreciated that one or more componentsof the fluidic system 200 may be modified to incorporate aluminium whichis exposed to ink supplied to the printhead.

Likewise, the nozzle assembly shown in FIG. 2 may be modified toincorporate a layer of aluminium metal 294 inside the nozzle chamber 24.A suitably modified nozzle assembly is shown in FIG. 9 having a layer ofaluminium metal 294 deposited in the pit 8 below the heater element 29.However, it will be appreciated that aluminium may be incorporatedanywhere inside the nozzle chamber 24, or indeed the print cartridge 202comprising the printhead and ink manifold.

An advantage of incorporating aluminium into the nozzle chamber 24 isits proximity to the silicon nitride structures. A disadvantage ofincorporating aluminium into the nozzle chamber 24 is that it requiresmodification of established MEMS fabrication processes, albeit arelatively minor modification which does not significantly change thenozzle design.

2. BPSG Corrosion

The corrosiveness of certain dye-based inks towards BPSG wasinvestigated. Further, the advantageous effects of metal additives insuppressing this corrosion were demonstrated.

BPSG coupons were prepared analogously to the silicon nitride couponsdescribed above. The BPSG coupons were exposed to a variety of dye-basedinks using the same methodology described above in connection withsilicon nitride coupons.

2.1 Colored Ink BPSG Corrosion Rates

Initially, the range of colored ink formulations were tested for theirBPSG corrosion rates. Table 10 shows the BPSG corrosion rates for theinks tested.

TABLE 10 BPSG Corrosion Rates for Colored Inks Test No. Test FluidCorrosion Rate (Angstroms per hr) 55 Cyan Ink 1 41.84 56 Magenta Ink 138.87 57 Yellow Ink 1 30.25 58 Black Ink 1 32.20

From Table 1, it can be seen that all inks tested were corrosive towardsBPSG, consistent with SEM microscopy observation on actual printheads.

2.2 Effect of Soluble Al(III) on BPSG Corrosion Rates

Table 11 shows the corrosions rates for the range of colored inks whenspiked with varying amounts of aluminium by the addition ofwater-soluble aluminium nitrate nonahydrate.

TABLE 11 Effect of Soluble Al(III) Ions on BPSG Corrosion Rates forCyan, Magenta, Yellow and Black Inks Corrosion Rate Test No. Test Fluid(Angstroms per hr) 55 Cyan Ink 1 41.84 59 Cyan Ink 1 + 1 ppm Al 40.99 60Cyan Ink 1 + 2 ppm Al 28.52 61 Cyan Ink 1 + 5 ppm Al 13.19 62 Cyan Ink1 + 10 ppm Al 13.57 63 Cyan Ink 1 + 20 ppm Al −0.97 64 Cyan Ink 1 + 50ppm Al −0.09 65 Cyan Ink 1 + 100 ppm Al −0.95 56 Magenta Ink 1 38.87 66Magenta Ink 1 + 1 ppm Al 5.58 67 Magenta Ink 1 + 2 ppm Al −0.36 68Magenta Ink 1 + 5 ppm Al −0.05 69 Magenta Ink 1 + 10 ppm Al 0.28 70Magenta Ink 1 + 20 ppm Al −1.08 71 Magenta Ink 1 + 50 ppm Al −2.04 72Magenta Ink 1 + 100 ppm Al −3.49 57 Yellow Ink 1 30.25 73 Yellow Ink 1 +1 ppm Al −0.23 74 Yellow Ink 1 + 2 ppm Al −0.10 75 Yellow Ink 1 + 5 ppmAl −0.42 76 Yellow Ink 1 + 10 ppm Al −1.29 77 Yellow Ink 1 + 20 ppm Al−0.43 78 Yellow Ink 1 + 50 ppm Al −0.07 79 Yellow Ink 1 + 100 ppm Al−0.26 58 Black Ink 1 32.20 80 Black Ink 1 + 1 ppm Al −0.01 81 Black Ink1 + 2 ppm Al 1.18 82 Black Ink 1 + 5 ppm Al 0.64 83 Black Ink 1 + 10 ppmAl −0.39 84 Black Ink 1 + 20 ppm Al 0.04 85 Black Ink 1 + 50 ppm Al−0.14 86 Black Ink 1 + 100 ppm Al −0.51

As evidenced by the data in Table 11, the corrosiveness towards BPSG ofall inks tested was minimized by the addition of relatively smallquantities of aluminium. At 20 ppm aluminium and above, all inks testedwere rendered benign towards BPSG.

2.3 Effect of Soluble Fe(III) on BPSG Corrosion Rates

Table 12 shows the corrosions rates towards BPSG for the range ofcolored inks when spiked with varying amounts of iron by the addition ofwater-soluble ammonium iron sulfate.

TABLE 12 Effect of Soluble Fe(III) Ions on BPSG Corrosion Rates forCyan, Magenta, Yellow and Black Inks Corrosion Rate Test No. Test Fluid(Angstroms per hr) 55 Cyan Ink 1 41.84 87 Cyan Ink 1 + 1 ppm Fe 41.79 89Cyan Ink 1 + 5 ppm Fe 0.56 90 Cyan Ink 1 + 10 ppm Fe 0.26 91 Cyan Ink1 + 20 ppm Fe 4.17 92 Cyan Ink 1 + 50 ppm Fe −0.80 93 Cyan Ink 1 + 100ppm Fe −1.99 56 Magenta Ink 1 38.87 94 Magenta Ink 1 + 1 ppm Fe 43.51 96Magenta Ink 1 + 5 ppm Fe 44.90 97 Magenta Ink 1 + 10 ppm Fe 40.24 98Magenta Ink 1 + 20 ppm Fe 17.05 99 Magenta Ink 1 + 50 ppm Fe 8.97 100Magenta Ink 1 + 100 ppm Fe −1.15 57 Yellow Ink 1 30.25 101 Yellow Ink1 + 1 ppm Fe 0.84 103 Yellow Ink 1 + 5 ppm Fe 0.17 104 Yellow Ink 1 + 10ppm Fe 0.59 105 Yellow Ink 1 + 20 ppm Fe 0.04 106 Yellow Ink 1 + 50 ppmFe −0.13 107 Yellow Ink 1 + 100 ppm Fe −0.35 58 Black Ink 1 32.20 108Black Ink 1 + 1 ppm Fe 21.09 110 Black Ink 1 + 5 ppm Fe −0.22 111 BlackInk 1 + 10 ppm Fe 0.98 112 Black Ink 1 + 20 ppm Fe 2.91 113 Black Ink1 + 50 ppm Fe 0.30 114 Black Ink 1 + 100 ppm Fe −0.28

As evidenced by the data in Table 12, the corrosiveness towards BPSG ofall inks tested was minimized by the addition of relatively smallquantities of iron. At 100 ppm iron, all inks tested were renderedbenign towards BPSG.

2.4 Effect of Different Fe(III) Salts on BPSG Corrosion Rates

Table 13 shows the corrosion rates towards BPSG for the cyan ink whenspiked with 100 ppm of Fe(III) from different soluble iron salts.

TABLE 13 Effect of Different Fe(III) Salts on BPSG Corrosion Rates forCyan Ink Corrosion Rate Test No. Test Fluid (Angstroms per hr) 55 CyanInk 1 41.84 93 Cyan Ink 1 + 100 ppm Fe −1.99 (ammonium iron sulfate) 116Cyan Ink 1 + 100 ppm Fe −1.52 (iron sulfate heptahydrate) 117 Cyan Ink1 + 100 ppm Fe −0.46 (iron chloride) 118 Cyan Ink 1 + 100 ppm Fe −0.72(iron bromide)

Table 13 demonstrates that the cyan ink was rendered benign towards BPSGby the addition of 100 ppm Fe(III), irrespective of the source of theFe(III) ions.

2.5 Effect of Other Metal Salts on BPSG Corrosion Rates

As foreshadowed above, it was postulated that the mechanism of corrosioninhibition relied on the formation of insoluble phosphate salts at theBPSG surface, which form a passivating layer that protects the BPSG fromcorrosion. To this end, a number of other metal additives were surveyedin accordance with their known metal phosphate solubilities.

Table 14 shows the corrosions rates towards BPSG for the magenta inkwhen spiked with 100 ppm of metal from a variety of different metalsalts. The solubility of the corresponding metal phosphate is also shownin Table 14, (The results from Test Nos. 56, 72 and 100 have beenincluded in Table 14 by way of comparison)

TABLE 14 Effect of Different Metal Salts on BPSG Corrosion Rates forMagenta Ink Corrosion Metal Rate Test Phosphate (Angstroms No. TestFluid Solubility (g/L) per hr) 56 Magenta Ink 1 38.87 72 Magenta Ink 1 +100 ppm Al 9.42 × 10⁻⁹ −3.49 (aluminium nitrate nonhydrate) 100 MagentaInk 1 + 100 ppm Fe 2.99 × 10⁻⁶ −1.15 (ammonium iron sulfate) 119 MagentaInk 1 + 100 ppm Cu 3.17 × 10⁻⁶ −0.23 (copper(II) nitrate hydrate) 120Magenta Ink 1 + 100 ppm Bi  1.10 × 10⁻¹⁰ −0.67 (bismuth(III) nitratepentahydrate) 121 Magenta Ink 1 + 100 ppm Mg 1.19 × 10⁻³ −0.13(magnesium nitrate hexahydrate) 122 Magenta Ink 1 + 100 ppm Ag 5.32 ×10⁻³ 0.62 (silver nitrate) 123 Magenta Ink 1 + 100 ppm Na 2.24 2.09(sodium nitrate)

From Table 14, it can be seen that the most effective metal additivesfor suppressing BPSG corrosion have highly insoluble phosphates.Notably, aluminium, iron, copper and bismuth salts were all highlyeffective in suppressing BPSG corrosion. Moreover, examination of theBPSG surface after exposure to these inks revealed a smooth glossysurface, indicating that a uniform passivation layer had formed on theBPSG surface.

On the other hand, exposure of the BPSG surface to inks spiked withsilver and sodium salts tended to result in a more pitted, non-uniformBPSG surface. This may indicate that the passivation layer was somewhatbrittle and less effective in suppressing corrosion.

2.6 Effect of Dye Counterions on BPSG Corrosion Rates

The inks tested contained off-the-shelf dyes supplied with a mixture ofcounterions. The evidence from metal phosphate solubility studies ledthe present Applicant to consider the nature of the dye counterion asbeing responsible for the corrosiveness of certain dyes. Specifically,it was postulated that counterions which form relatively solublephosphates (e.g. ammonium, potassium and sodium) would be more corrosivetowards BPSG than counterions which form relatively insoluble phosphates(e.g. lithium and tetramethylammonium). Further, the benefits of metaladditives in the ink would be best realized in inks containingcounterions which form relatively soluble phosphates.

Accordingly, Black Ink 2 and Magenta Ink 1 were modified by ion exchangeto produce inks containing only a single counterion. Table 15 shows theBPSG corrosion rates for mono-counterion black inks in the presence ofvarying amounts of an aluminium additive (aluminium nitratenonahydrate). Table 16 shows the BPSG corrosion rates formono-counterion magenta inks in the presence of varying amounts of thealuminium additive.

TABLE 15 Effect of Dye Counterions in Black 2 Ink on BPSG CorrosionRates Corrosion Rate Test No. Mono-Counterion Metal Additive (Angstromsper hr) 124 Potassium None 57.53 125 Sodium None 41.39 126 Lithium None5.43 127 TMA None 0.02 128 Potassium  1 ppm Al 10.89 129 Sodium  1 ppmAl 0.36 130 Lithium  1 ppm Al 2.66 131 TMA  1 ppm Al −0.64 132 Potassium10 ppm Al 0.80 133 Sodium 10 ppm Al −0.24 134 Lithium 10 ppm Al 135 TMA10 ppm Al 0.34 136 Potassium 50 ppm Al −1.23 137 Sodium 50 ppm Al −1.29138 Lithium 50 ppm Al −0.37 139 TMA 50 ppm Al −0.62 140 Potassium 100ppm Al  −1.37 141 Sodium 100 ppm Al  −4.17 142 Lithium 100 ppm Al  −1.36143 TMA 100 ppm Al  −0.66

TABLE 16 Effect of Dye Counterions in Magenta 1 Ink on BPSG CorrosionRates Corrosion Rate Test No. Mono-Counterion Metal Additive (Angstromsper hr) 144 Ammonium None 40.79 145 Potassium None 32.45 146 Sodium None30.16 147 Lithium None 0.30 148 TMA None 0.24 149 Ammonium  1 ppm Al14.73 150 Potassium  1 ppm Al 6.60 151 Sodium  1 ppm Al −0.07 152Lithium  1 ppm Al −0.05 153 TMA  1 ppm Al −0.31 154 Ammonium 10 ppm Al15.42 155 Potassium 10 ppm Al −0.30 156 Sodium 10 ppm Al −0.25 157Lithium 10 ppm Al −0.22 158 TMA 10 ppm Al −0.47 159 Ammonium 50 ppm Al−0.65 160 Potassium 50 ppm Al −0.52 161 Sodium 50 ppm Al −0.15 162Lithium 50 ppm Al −0.77 163 TMA 50 ppm Al −3.01 164 Ammonium 100 ppm Al −1.20 165 Potassium 100 ppm Al  −6.85 166 Sodium 100 ppm Al  −16.67 167Lithium 100 ppm Al  −1.56 168 TMA 100 ppm Al  −6.78

The results in Tables 15 and 16 both demonstrate that the dye counterionis a significant factor controlling BPSG corrosion rates. The evidencefrom these experiments shows an order of BPSG corrosivity as follows:

Ammonium>Potassium>Sodium>>Lithium>TMA (tetramethylammonium)

A key finding is that inks containing potassium ions are highlycorrosive towards BPSG. Furthermore, this corrosivity can be suppressedwith the addition of sufficient quantities of a metal additive, such asaluminium nitrate.

Commercial inkjet dyes are often supplied with a mixture of dyecounterions. Dyes containing potassium counterions in significantamounts are expected to be highly corrosive towards BPSG, whereas thosecontaining, for example, only lithium ions are expected to be much lesscorrosive towards BPSG. The dye counterions have an important role inthe overall balance of ink formulations. In many instances, it is notpossible simply to switch to a less corrosive counterion (e.g. lithium)because this affects the solubility of the dye and may cause undesirableside-effects, such as precipitation of the dye in a printhead. It is anadvantage of the present invention that inks containing corrosivecounterions can be used without changing the composition of the dye. Theaddition of metal additives to the ink formulation in relatively smallquantities has been shown to be remarkably effective in suppressingcorrosion of BPSG structures.

3. Silicon Oxide Corrosion

The corrosiveness of a dye-based ink towards silicon oxide wasinvestigated. Further, the advantageous effects of metal additives insuppressing this corrosion was demonstrated.

Silicon oxide coupons were prepared analogously to the silicon nitridecoupons described above. The silicon oxide coupons were exposed to avariety of dye-based inks using the same methodology described above inconnection with silicon nitride coupons.

3.1 Effect of Al(III) on Silicon Oxide Corrosion Rates

The cyan ink formulation was tested for its silicon oxide corrosion ratewith and without an aluminium nonahydrate additive. The results areshown in Table 17.

TABLE 17 Silicon Oxide Corrosion Rate for Cyan Ink Test No. Test FluidCorrosion Rate (Angstroms per hr) 169 Cyan Ink 1 3.55 170 Cyan Ink 1 +10 ppm Al −1.00

From Table 17, it can be seen that the cyan ink has some corrosivitytowards silicon oxide, albeit less than its corrosivity towards siliconnitride and BPSG. The additional of 10 ppm Al(III) was sufficient tosuppress this corrosivity completely.

It will, of course, be appreciated that the present invention has beendescribed by way of example only and that modifications of detail may bemade within the scope of the invention, which is defined in the claimsappended hereto.

1-20. (canceled)
 21. An ink set for an inkjet printer, the ink setcomprising different colored aqueous inks having differentconcentrations of potassium ions, wherein: first ink comprising a firstcolorant contains a relatively lower concentration of potassium ionsthan a second ink comprising a second colorant; and the second inkcontains an Al(III) additive in a higher concentration than the firstink.
 22. The ink set of claim 21, wherein the second colorant is an azodye having sulfonate groups.
 23. The ink set of claim 21, wherein theprinter comprises a printhead having exposed corrodible structures. 24.The ink set of claim 22, wherein the exposed corrodible structures areselected from the group consisting of: silicon nitride,borophosphosilicate glass (BPSG) and silicon oxide.
 25. The ink set ofclaim 21, wherein the second ink contains the Al(III) additive in anamount ranging from 0.01 to 200 ppm.
 26. The ink set of claim 21,wherein the first ink does not contain any Al(III) additive.